A number of electronic systems require frequency conversion or frequency synthesis. For example, radio frequency (RF) "downconverters" receive a transmitted RF modulated carrier waveform at a high frequency (e.g., 1.9 GHz) and frequency convert the modulated signal to an unmodulated signal at a lower frequency (e.g., 250 MHz). RF receivers are increasingly being used in a number of popular "wireless" handheld, low power devices, such as Personal Communications Service (PCS) and Personal Handiphone Service (PHS) cellular telephones. Most of the functions of a complete RF receiver can now be fabricated in an integrated circuit (.degree.C.) or IC chip set. In implementing an RF integrated circuit (RFIC) receiver for low power use, it was found by the inventors that there was a need for additional gain in such a receiver without expending additional DC current.
Another problem of implementing an RFIC was found to be in implementing a balun function. A balun (short for BALanced to Unbalanced) is a transformer connected between a balanced source or load (signal line) and an unbalanced source or load (signal line). A balanced line has two signal line conductors, with equal currents in opposite directions. The unbalanced signal line has just one conductor; the current in it returns via a common ground or earth path. Typically, an RF balun function is implemented as an off-chip transformer or as a quarter wave hybrid (lumped or microstrip) integrated into an RFIC. These RF baluns are commonly used with doubly balanced mixers such as a Gilbert cell and a passive FET ring. Although such baluns consume no current, these passive structures are lossy, narrowband, and consume precious printed circuit board or RFIC area A differential amplifier is another alternative solution, but at RF frequencies, the noise figure is prohibitive. As another alternative, in many applications, a balun is not used, and a passive FET ring or a Gilbert cell mixer is driven single-ended at RF, which degrades the balance and performance of the mixer. One attempt to address these issues with an active circuit is disclosed in Gilbert, "The MICROMIXER: A Highly Linear Variant of the Gilbert Mixer Using a Bisymmetric Class-AB Input Stage", IEEE Journal of Solid-State Circuits, Vol. 32, No. 9 (Sept. 1997).
The inventors have determined that there is a need for a circuit that can add gain in an RFIC frequency converter without using additional DC current, and a circuit for implementing an on-chip balun function without the drawbacks of prior circuits. The present invention meets both needs with a single circuit.